In Taralp U.S. Pat. No. 4,156,173 issued May 22, 1979 and entitled "Input Impedance Matching Of A Bipolar Transistor Amplifier Employing A Coaxial Transformer", there is described a common emitter bipolar transistor amplifier for signals in the VHF (30 to 300 MHz) range in which a coaxial transformer is used for impedance matching between the amplifier and the signal source. The signal source typically has an impedance of 50 .OMEGA., whereas the base-emitter circuit of the bipolar transistor of the amplifier typically has an impedance which is much higher than this. Accordingly, the coaxial transformer has a relatively high turns ratio, for example 5:1, to provide a desired high (25:1) impedance transformation ratio.
As described in U.S. Pat. No. 4,156,173, the coaxial transformer comprises a first winding on a ferrite toroidal core which is substantially surrounded by a metal enclosure which constitutes a single turn second winding of the transformer. A transformer of this general type is also described in Silverstein U.S. Pat. No. 3,353,130 issued Nov. 14, 1967 and entitled "High Ratio Vacuum Tube Input Transformer". The coaxial transformer provides both the high turns ratio which is necessary for impedance matching of the input of the amplifier, and tight coupling between the transformer windings which is necessary for the required bandwidth of the amplifier.
U.S. Pat. No. 4,156,173 is concerned only with impedance matching of the input of the amplifier, which contributes to low noise. It does not discuss other characteristics of the amplifier such as amplifier efficiency, linearity, and isolation between the input and output of the amplifier. In addition, the patent illustrates a load in the collector circuit of the transistor, but does not describe the nature or impedance of this and does not address impedance matching between the amplifier and the load.
Isolation between the output and the input of a wideband amplifier is particularly important in view of the situations in which such an amplifier may be used. For example, such an amplifier may be used to drive a transmission line or with a following mixer circuit, either of which may reflect signals back to the amplifier output. Unless the amplifier has very good isolation between its output and input circuits, such undesired reflected signals will be communicated at least in part to the input circuitry of the amplifier and cause seriously degraded performance (e.g. magnitude and phase characteristics, and to a lesser extent noise figure) of the amplifier.
Linearity of a wideband amplifier is also very important, it being desirable for a wideband amplifier to have a constant gain for all input signal levels over the wide range of operating frequencies, or bandwidth, of the amplifier. In addition, a relatively high amplifier efficiency (for a Class A amplifier) is desirable in order to reduce power consumption and dissipation, especially when such amplifiers are used in densely packed circuitry such as is increasingly common using surface-mount technology.
An object of this invention, therefore, is to provide an improved wideband bipolar transistor amplifier.
To this end, the invention utilizes a dual-emitter bipolar transistor arrangement in conjunction with two high-ratio transformers for maximum signal coupling at the input and output port.